.-. How is Mei beating gaara .-. Or onooki
And how is tsuande beating onooki or gaara .-.
Ok.
Let's do this.
First off, we're going to assume a couple things.
We're going to assume that:
-> The kages do NOT know about each other's special abilities.
-> In fact, the kages do NOT know about each other at all.
-> We assume that no kage will always be able to maintain a defense and offense 100 % of the time. In other words, there will be times when each kage will be susceptible to an attack.
This helps establish a false winner/false loser in the battle who will turn out for good reason to be the true winner/true loser. As we know, assumptions have shaped and changed the outcomes of one on one fights.
So, You said "How is Mei beating Gaara"... I say:
She opens with hidden mist followed by acid boil release. Gaara is dependent on his sand for offense and defense. If he doesn't have chakra he cannot use his sand, which is really the unfortunate thing about gaara... he doesn't have any other arsenal. By using hidden mist, gaara will be unable to sense her. By using acid boil release, the air molecules will eat away at gaara's chakra, draining him, making him susceptible to lava release, especially if it is done at a wide range and instantaneously.
In fact, Mei is the epitome of a perfect counter for gaara.
If we look at onooki, it is the same thing. He relies on massive chakra to use his kekkai touta (jinton release). Without chakra levels high enough, he is susceptible. Essentially, Mei's acid boil technique, which eats away at a user's chakra level is the perfect counter to anyone who has major ninjutsu based attacks and defenses.
As an aside, the reason why it wouldn't work against EDO-madara is because tere is no point eating away at a dead person's chakra who can regenerate that over and over again, it would just affect the other kages who are fighting with her. (just threw that in there to counter the people who would try to argue that she is weak because she couldn't defeat edo madara with that).
Now, why would Tsunade for example, be a perfect counter to mei?
Simple reason: Tsunade can regenerate the damage done by mei's acid release. It's only because of tsunade's rapid instant regeneration that i place her above mei, otherwise mei would be above her. Couple her instant regen. with her strength and that would make her a perfect counter mei's ninjutsu and long range arsenal. Tsunade gets close, slam bam thank you mam and mei's done.
You then ask "How is Tsunade beating onooki or gaara?"
Gaara would have the advantage at the start of the fight using his sand to keep tsunade at bay, however, he would eventually get tired as Tsunade's chakra level is much much much higher than gaara's, and that's not even bringing in the seal activation. As soon as gaara lowers his offense and prepares for his defense, tsunade would have the chance to break through the barrier and catch him off guard.
What would gaara do if Tsunade was caught off guard? Tickle her with his sand? She would regenerate any damage due to friction or impact.
You also ask "How is tsunade beating Onooki?"
People argue that onooki's jinton is at an atomic level. I argue that Tsunade's regeneration is also at an atomic level. Here's the proof:
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"The speed will rip it to shreds." It's an instant teleportation jutsu. Atomic level none the less. I specialize in medical physics so when something is instant, that means it's in the range of 10^-6 to 10^-16 seconds. That range. Atoms are at the nano level, which is 10^-9 meter range. Atmoic level speed means approx. 10^-9 seconds, which falls in the range of instant. We can assume that the instant teleportation was ripping her body at an atomic level. Her jutsu was able to keep her alive and give her strength.
Hence why she counters onoki's strongest jutsu - jinton.
Anyways, thank you very much and have a good day!